A reheat burner or second burner of a sequential combustion gas turbine can include a tubular mixing zone (for example, having a quadrangular or trapezoidal cross section) with a lance for injecting a fuel projecting thereinto.
EP2072899 discloses a lance for a reheat burner having a body with a first duct with first nozzles for a liquid fuel, a second duct with second nozzles for a gaseous fuel and a third duct with third nozzles for shielding air. For example, the third duct can encircle the second duct that, in turn, can encircle the first duct.
In this known lance, the nozzles are coaxial and, thus, their outlets can all be located at the same position.
During operation, while hot gases (coming from an upstream combustion chamber and turbine) pass through the tubular mixing zone, fuel (liquid or gaseous fuel) can be injected into the same mixing zone via the lance. Because of high temperature of the hot gases, after injection the fuel heats and after a prefixed time delay (depending on the particular fuel), it can start to spontaneously burn.
Nevertheless the features of liquid and gaseous fuel can be different and the delay time of a gaseous fuel can be longer than the delay time of a liquid fuel.
Because nozzles for liquid and gaseous fuel are coupled in nozzles groups (i.e., their outlets are all located at the same position), the dimensions and proportions of the lance and nozzles may not be optimized but have to suffer the constraints deriving from both liquid and gaseous fuels.
For this reason, liquid fuel can be injected together with water (i.e., when operating with liquid fuel a mixture of fuel and water is injected in the burner), in order to increase the ignition delay time to an amount allowing the correct operation of the burner. This can prevent the liquid fuel from starting to burn in the burner mixing zone, before it enters the downstream combustion chamber.
For these reasons, operation with liquid fuel could be very expensive, because in some places water is expensive.
EP 0 594 127 discloses a burner with a lance having a body with a first duct for injecting a liquid fuel and a second duct for injecting a gaseous fuel. These ducts have nozzles whose outlets are apart from each other.